Sounds like a fun trip! I took my '01 from near STL to Rockville, MD last summer. I went 64-> 79-> 68. Nice drive, and the best tank the Boxster returned was 34 mpg. I opted to have the top up for most of the drive... 900 miles of sticky humidity isn't my idea of a fun drive.
The practical sports car... fit my bike, riding gear, camera gear, and clothes for a week in the thing with room to spare:
As far as hotels in STL are concerned, depends on the part of town... just avoid East St. Louis! Maybe check out
http://www.hotels-stl.com/ for some ideas.
Oh, if you wouldn't normally do this, avoid Arby's as well. Nothing like being stuck in the middle of Illinois in a traffic jam with a grumble down under from a bad batch of roast "beef".